When the heart or another muscle contracts, the body generates a very low amplitude electric signal known as a biopotential signal. It is well known that this biopotential signal can be electronically detected on the surface of a person's skin. Because the heart expands and contracts in a regular rhythm, it generates a periodic biopotential signal. Thus, certain periodic voltage fluctuations on a person's skin correspond to heartbeat.
Devices for monitoring the heart rate of a human or animal are well known. Examples of these types of devices are described in U.S. Pat. Nos. 4,248,244; 4,540,001; and 4,898,182. Generally, most heart rate monitors use electrodes to sense voltage fluctuations on a person's skin. The signal thus sensed is then amplified and passed through a suitable filter for filtering out the biopotential signals unrelated to heart rate. The frequency of the residual signal is then determined and displayed as heart rate (in beats per minute).
Under certain ideal conditions, such as found in a hospital, the heart-related biopotential signal can be easily separated from other signals. During physical exercise, however, biopotential signals generated from the movement of muscles other than the heart tend to mask the heart's biopotential signal. Particularly troublesome are repetitive exercises, such as riding an exercise bike. In repetitive exercises, muscles tend to be exerted in a rhythmic or periodic fashion, thereby generating a periodic biopotential signal which cannot easily be distinguished from heart rate.
Equally problem prone is the interface between the human subject and the electrodes which are used to detect biopotential signals. In practice, persons exercising prefer to not be encumbered by the types of bracelet, harness or clip which are typically used to make contact with the electrodes. Ideally, the electrodes are placed on a handlebar or other location which the person touches as a matter of course while exercising. Under these conditions, however, the person is likely to remove his or her hands from time to time, thereby complicating efforts to accurately and continuously monitor heart rate.
One example of deficiencies in the prior art is provided by U.S. Pat. No. 4,248,244, issued to Charnitski et al. Charnitski teaches the use of electrodes which are connected to an electronic circuit for producing a burst of alternating electronic signal in response to biopotential impulses. The electronic circuit in turn is connected to a peak detector, which detects the peak amplitude of each burst of alternating signals and produces an electric pulse related in time thereto. A computer is used to measure the time between pulses and to selectively convert the measured times to a number representing heart rate. In practice, however, the arrangement disclosed Charnitski is unable to distinguish heart rate from other periodic muscular movements (such as pedaling a bicycle).